The present invention pertains generally to flue gas dampers and more particularly to disk dampers.
With the advent of the increased use of pulverized coal in power plants as a result of increased price in petroleum based fuels, there has been an increased need for devices to protect the environment, such as flue gas scrubbers, bag houses, precipitory sprayers and associated bypasses which require extensive use of large reliable dampers. The increased use of large dampers has also been necessitated by an increase in power plant size, a general increase in complexity, the need to isolate portions of ducting for the purpose of inspection and repair, and the economic aspects of providing heat recovery systems to increase efficiency of the overall system. Larger dampers are preferable to smaller dampers since larger dampers produce less pressure drop across the damper surface and allow use of components which are more economical from a capital investment and operational standpoint. For example, large bypass dampers used in conjunction with bag houses in a pulverized coal plant can greatly reduce the specifications of a stack exhaust fan, and consequently greatly improve the economies of a power plant from a capital investment and operational standpoint. Additionally, the ability of a damper to provide a tight seal can greatly reduce heat loss in certain applications and consequently further improve the economics of the power plant system.
However, various problems exist in producing large dampers which provide a large opening, are reliable in operation, require low maintenance, have a long lifetime, are fast acting, erosion resistant and capable of providing a tight seal over a wide range of temperatures. Various dampers have been designed to meet these requirements and provide both control and isolation, including the louver damper, guillotine damper, butterfly damper, swinging blade damper, and the disk or poppet damper.
The louver type damper consists of several louver blades mounted in a parallel manner across a duct with a centrally pivoted shaft extending through a frame and driven by a linkage. Louver dampers are versatile in that they fit in most ducting at any attitude, are compact and require little external clearance, are lightweight, provide a drive which is readily accessible, have a simple control function, provide a quick response to thermal transients, are fast opening and closing, provide low leakage to the outside environment, and require low actuation power. However, the disadvantages of the louvered damper are its large leakage perimeter, high leakage rates through the seals in a closed position, high pressure drop due to blade and seal obstruction of the flow, and inherent problems which exist because the blades and seals are always disposed in the gas flow. For example, the blades tend to corrode, catch ash and waste material from the scrubber, and produce a fluttering motion resulting from the flimsiness of the long thin blades in the gas flow causing buckling and warping of the blades and, leakage and jamming of the louvered damper.
Guillotine type dampers are capable of providing isolation to permit inspection, maintenance and repair of items, providing a low pressure drop across the damper, and resistance to damage from surges in furnace pressure. However, guillotine dampers are slow moving, require large drive mechanisms due to the heavy weight of the blade, require a large structure outside the duct to support the drive mechanism and retracted blade, are susceptible to wear and corrosion in the drive mechanism, are sensitive to attitude, are difficult to unjam and are unable to modulate gas flow.
Butterfly or wafer dampers are light weight, simple, quickly actuated, and often comparatively low in cost. The disadvantages in the butterfly damper include high leakage, blade flutter, tendency of the blade to warp, and the necessity for a large clearance for the open blade. The frame of butterfly dampers is normally light, so twisting and distortion can easily occur.
Swinging blade dampers, are simple and can withstand high temperatures and the blade is located out of the gas flow so erosive particles do not impact directly on the surface of the swinging blade directly. The disadvantages of the swinging blade damper include the requirement of a large frame and drive mechanism, and flutter of the blade when moving across the flow or splitting a flow between outlets. The swinging blade damper can be attitude sensitive and requires special seals along side portions.
The conventional disk or poppet type damper is fast actuating and can provide and initial slow opening stroke which is particularly useful in baghouse operations to prevent damage to the bags. This type of damper uses a metal to metal contact to form a seal between a seat and flexible disk edge. For large diameter disk dampers, flexible disks are incapable of providing a tight seal due to inability of large disks to deform sufficiently to accommodate large displacements resulting from inherent warpage of the disk and seat. This type of damper is employed over a wide range of temperatures, i.e., room temperature to approximately 450.degree. in pulverized coal plants, and up to 950.degree. in gas turbine plants. This wide range of temperatures results in inherent warpage of the damper seat, which is especially prevalent in large opening disk type dampers, such as those having a diameter greater than 4 feet. The use of flexible materials such as silicons has been ineffective since these materials tend to brittlize and are incapable of maintaining a tight seal across high pressure differentials. For diameters greater than 4 feet, additional problems exist since normal sheet metal is fabricated in widths of 4 feet. Additional warpage is encountered during fabrication due to the necessity to weld sheets together to accommodate openings larger than 4 feet.
If a flexible disk is attempted to be deformed sufficiently to accommodate large displacements resulting from inherent warpage of the seat, the yield point of the metal of the flexible disk can be exceeded to cause permanent deformation. In other words, normally the yield strength of the flexible disk is insufficient to prevent permanent deformation to accommodate the displacements of the seat due to inherent warpage. The yield point is defined as the minimum unit stress at which a structural material will deform without an increase in the load, while the yield strength is defined as the unit stress corresponding to a specific amount of permanent unit deformation. Regular carbon steels have been incapable of providing sufficient deformation to provide a seal for large diameter flexible disk dampers, i.e. greater than 4 feet, without causing permanent deformation of the disk because of inherent warpage in the seat due to the wide range of temperatures at which disk dampers are employed. In other words, flexible disks using conventional carbon steels are incapable of compensating for normal warpage in the disk damper seat for large disk diameters.
In an attempt to overcome the problems of inherent warpage in the damper seat, grinding of the damper seat has been employed in prior art devices in an attempt to reduce the necessity of large displacements of flexible disk. Additionally, anti-rotation guides have been placed on the disk spindle to make certain that the disk will repeat its positioning location on the disk seat for each closing. In this manner, the circumferential orientation of the flexible disk in the annular seat remains fixed.
However, warpage of the damper seat can have a large variation as a result of the wide range of temperatures in which the damper is used. Consequently, it is difficult to determine which areas should be ground to provide a seal at full operating temperature. Moreover, seats which could be ground to provide a seal at full operating temperature may not provide a sufficient seal at lower temperatures, and consequently severely stress the utility of the damper. Moreover, the process of grinding of the damper seat is expensive and time consuming, and results in a damper which, in many cases, is marginally effective.